1. Field of the Invention
This invention relates to an overload protecting apparatus for protecting a press from damage to the members thereof caused by an overload.
2. Description of the Prior Art
Generally speaking, in a mechanical press, the pressure generated around the lower dead center of a slide is so high that the members of the press are likely to be damaged when the pressure exceeds a nominal pressure equal to a maximum pressure which the members of the press can stand. In order to protect the members of the press from such damage, the mechanical press is provided with an overload protecting apparatus.
FIG. 1 is a system diagram showing a conventional hydraulic type overload protecting apparatus for a press. The overload protection apparatus is conventionally provided at a slide point where connecting rods are connected to a slide 1. Slide 1 represents, by way of example, a top press plate of the press.
More particularly, the apparatus comprises pistons 52 and 53 connected to piston rods 16 and 17 which are connected to connecting rods 50 and 51; and oil chambers 4 and 5. Chambers 4 and 5 are disposed at slide points 2 and 3 where a slide 1 is connected to piston rods 16 and 17 and thereby connecting rods 50 and 51 for moving the slide, or top press plate, of the press up and down. The pressure in the oil chambers is varied by movement of the pistons 52 and 53 so as to be in proportion to the load applied on the slide 1. The apparatus further includes a protector valve 8, having an oil chamber 6 in communication with the oil chambers 4 and 5. Working fluid, such as oil, in the oil chambers 4 and 5 is discharged via the oil chamber 6 to a drain 7 when the pressure of the oil in the oil chamber 6 exceeds a predetermined level. The protector valve 8 comprises a piston 9, the oil chamber 6 and an air chamber 10, the piston 9 partioning the protector valve 8 into the latter two. Air supplied from an air supply source (not shown) keeps the pressure inside the air chamber 10 at a predetermined level, thereby providing a constant upward force on the piston 9.
In the conventional overload protecting apparatus shown in FIG. 1, when overload is applied on the slide 1 so that the pressure of at least one of the oil chambers 4 and 5 exceeds the pressure of the protector valve 8 (i.e. the air pressure in the air chamber 10), the piston 9 of the valve 8 is lowered so as to communicate the oil chamber 6 with the drain 7, thereby discharging the oil in the oil chambers 4 and 5 into the drain 7. As a result, pistons 52 and 53 at the slide points 2 and 3 move closer to the slide 1, thereby releasing the overload. An oil pump 11 is provided for pumping oil via the solenoid valve 12 into the oil chambers 4 and 5, whereby in a normal state the pressure of the oil in chambers 4 and 5 is always pushing the slide 1 downwardly in FIG. 1 against the force of a balance cylinder (not shown). A relief valve 13 keeps the pressure of the oil pumped by the oil pump 11 at a predetermined level.
The numeral 14 in FIG. 1 designates a limit switch operated by the piston 9 of the protector valve 8 for protecting an overload operation. When the piston 9 of the protector valve 8 is raised, the limit switch 14 is activated to connect a power source to a solenoid 15 of the solenoid valve 12, thereby setting the solenoid valve 12 in the position to supply oil into the oil chambers 4 and 5. When the piston 9 of the protector valve 8 is lowered, the limit switch 14 disconnects the power source to solenoid 15 so as to demagnetize the solenoid 15, thereby setting the solenoid valve 12 in the position suitable for drainage so as to shut off the supply of the working oil into the oil chambers 4 and 5.
Incidentally, in the press comprising the overload protecting apparatus, causes generating the overload are removed after the overload is released by means of the overload protecting apparatus. Consequently, a reset switch, not shown, is turned on so as to excite the solenoid 15 of the solenoid valve 12, thereby setting the solenoid valve 12 in the position suitable for supplying the oil to chambers 4 and 5 in the normal state. Each base of the piston rods 16 and 17 is threaded for attachment to the piston 52 and 53 for adjusting their slide distance.
During the operation of a press, when an unbalanced load is applied on the slide 1, the slide 1 inclines, thereby decreasing accuracy of pressing operation and shortening the life spans of the upper mold and the lower mold of the press. In order to prevent the foregoing, when an unbalanced load is applied on the slide 1, it is necessary to detect the unbalanced load and to remove the causes thereof. However, the conventional overload protecting apparatus is so constructed that the oil chambers 4 and 5 are directly communicated with each other via the oil chamber 6 provided in the protector valve 8. Accordingly, even when an unbalanced load is applied, the pressures of both chambers come to the same level. Therefore, even though the pressures applied inside the oil chambers 4 and 5 are detected, it is impossible for the conventional type of overload protecting apparatus to detect an unbalanced load applied on slide 1.
Furthermore, the pressure of the air pumped into the air chamber 10 pushes up the piston 9 of the protector valve 8 in the conventional overload protecting apparatus. Accordingly, even when an overload is applied on the slide 1 and the pressure applied inside the oil chamber 6 pushes down the piston 9 of the protector valve 8, the piston 9 is not lowered down to the stroke end. As a result, all the working oil pumped into the oil pressure chambers 4 and 5 is not discharged into the drain 7. When an object to be pressed is sufficiently thick and is placed between the upper mold and the lower mold and an overload occurs, there is a possibility that the slide 1 cannot be raised up to the height suitable for removing the object.
Examples of the previously described conventional overload protecting apparatus are disclosed in Japanese Patent Laid-open Nos. 54-55874, 61-83000; Utility Model Laid-open No. 61-111700; and Patent Application Publication Nos. 42-12230, 53-34666, 59-33480. Each of these apparatus suffer the drawback noted above where an unbalanced load applied on a slide cannot be detected.